1. Field of the Invention
The present invention relates to an optical disc apparatus and a signal generation method. The present invention is suitably applied to, for example, an optical disc apparatus for reproducing information from an optical disc in which record marks are formed in a uniform recording layer.
2. Description of the Related Art
In the field of optical disc apparatuses, ordinary optical discs having signal recording layers, e.g., a CD (Compact Disc), a DVD (Digital Versatile Disc), and a Blu-ray Disc (registered trademark, hereinafter referred to as a “BD”), have hitherto been widely used. In such an optical disc apparatus, information is reproduced by irradiating a light (optical) beam to a target track in the signal recording layer where the light beam is to be irradiated (hereinafter referred to as a “desired track”), and by reading light reflected at the desired track.
In that type of related-art optical disc apparatus, when the light beam is irradiated to the signal recording layer, the irradiated light beam is reflected at the signal recording layer such that the reflected light can be received in predetermined or more quantity. In the related-art optical disc apparatus, therefore, various error signals indicating deviations of the light beam from the desired track (e.g., a focus error signal and a tracking error signal) are generated based on the reflected light, and servo control is executed so as to irradiate the light beam to the desired position.
Also, in that type of related-art optical disc apparatus, information is recorded by irradiating a light beam to the signal recording layer of the optical disc, thus changing, e.g., local reflectance of the signal recording layer.
In the optical disc, it is common knowledge that the size of a beam spot formed when the light beam is condensed (focused) through an objective lens, etc., is approximately expressed by λ/NA (λ: wavelength of the light beam and NA: numerical aperture) and resolution is also proportional to such a value. According to the BD technique, for example, data of about 25 GB can be recorded per layer of an optical disc with a diameter of 120 mm.
Optical discs are generally used to record various contents, e.g., music contents and image (video) contents, and various kinds of information, e.g., various data for use in computers. In recent years, particularly, an amount of information handled has increased with higher definition of images and higher sound quality of music, and an increase in the number of contents recorded on one optical disc has been demanded. Thus, an optical disc having a larger capacity has been demanded.
In view of those demands, an optical disc apparatus is proposed in which standing waves are recorded as record marks in a uniform recording layer of an optical disc by utilizing, e.g., a hologram, and the recording layer is formed plural in one disc for the purpose of simplifying the optical disc and increasing the disc capacity (see, e.g., Japanese Unexamined Patent Application Publication No. 2008-71433).
In the proposed optical disc apparatus, a light beam is irradiated to a mark layer in the optical disc, which includes record marks therein, and a return light beam returning from the optical disc is received. Further, in the proposed optical disc apparatus, information is reproduced by detecting the presence or the absence of the record marks based on the return light beam reflected at the record marks.